Histogram based optimization for optical modulation

ABSTRACT

The present invention is directed to communication systems and methods. In a specific embodiment, the present invention provides an optical receiver that receives a data stream from an optical transmitter. The optical receiver determines a histogram contour parameter using the data stream and inserts the histogram contour parameter into a back-channel data segment, which is then transmitted to the optical transmitter. The optical transmitter changes its data transmission setting based on the histogram contour parameter. There are other embodiments as well.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 16/828,820, filed Mar. 24, 2020, which is acontinuation of U.S. application Ser. No. 16/582,985, filed Sep. 25,2019, now U.S. Pat. No. 10,644,803, issued May 5, 2020, which is acontinuation of U.S. application Ser. No. 15/644,342, filed on Jul. 7,2017, now U.S. Pat. No. 10,469,176, issued Nov. 5, 2019, which areincorporated herein by reference in their entirety.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

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REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISK

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BACKGROUND OF THE INVENTION

The present invention is directed to communication systems and methods.

Over the last few decades, the use of communication networks exploded.In the early days of the Internet, popular applications were limited toemails, bulletin board, and mostly informational and text-based web pagesurfing, and the amount of data transferred was usually relativelysmall. Today, Internet and mobile applications demand a huge amount ofbandwidth for transferring photo, video, music, and other multimediafiles. For example, a social network like Facebook processes more than500 TB of data daily. To move a large amount of data, opticalcommunication networks are often used.

With high demand for communication networks came high demand for qualitynetworking devices. In high-speed communication systems, havingoptimized optical transceivers can meaningfully improve performance. Forexample, various parameters of optical transmitter, such as biasvoltages for modulator and laser devices, can be adjusted and optimizedin a communication system for improved performance.

Over the past, there have been various techniques for optimizingparameters and settings for optical transceivers. Unfortunately,existing techniques are inadequate for reasons explained below. Improvedmethods and systems for optimizing optical communication devices aredesired.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to communication systems and methods.In a specific embodiment, the present invention provides an opticalreceiver that receives a data stream from an optical transmitter. Theoptical receiver determines a histogram contour parameter using the datastream and inserts the histogram contour parameter into a back-channeldata segment, which is then transmitted to the optical transmitter. Theoptical transmitter changes its data transmission settings based on thehistogram contour parameter. There are other embodiments as well.

According to an embodiment, the present invention provides an opticaltransceiver apparatus. The apparatus includes an optical receiver forconverting incoming optical signals to incoming electrical signals. Theapparatus also includes a digital signal processor (DSP) configured toanalyze the incoming electrical signals and to generate a histogramcharacterizing the incoming electrical signals. The apparatus furtherincludes a forward-error correction (FEC) module configured encode theincoming electrical signal. The apparatus also includes a controlmodule, which processes the incoming electrical signal and to generate asignal quality value based on the histogram. The signal quality value iscalculated using a quadratic fit of the histogram. The control module isfurther configured to generate back-channel data based at least on thesignal quality value. The control module further is configured to insertthe back-channel data to an outgoing data stream. The apparatus has anoptical transmitter for generating output optical signals based on theoutgoing data stream.

According to another embodiment, the present invention provides ancommunication system. The system includes a communication link. Thesystem includes a first transceiver comprising a first control moduleand a first transmitter. The system further includes a secondtransceiver comprising a second control and a second transmitter. Thesecond transceiver is configured to send data to the first transceiver.The first transceiver is configured to process a first data streamreceived from the second transceiver and to detect a first back channeldata. If the first transceiver detects the first back channel data, thefirst transceiver is configured to determine a histogram contourparameter associated with the first data stream and insert the histogramcontour parameter into a second back channel data. The second backchannel data is embedded in a second data stream. The second transceiveris configured to process the second data stream received from the firsttransceiver. The second transceiver is configured to detect the secondback channel data and adjusts one or more operating parameters based onthe histogram contour parameter.

According to yet another embodiment, the present invention provides amethod for optimizing optical communication. The method includestransmitting a first data stream from a first optical transceiver to asecond optical transceiver via an optical communication link. The methodalso includes detecting a first back-channel data segment at the firstdata stream by the second optical transceiver. The method furtherincludes determining a histogram contour parameter associated with thefirst data stream by the second optical transceiver. The method alsoincludes generating a second back-channel data by the second opticaltransceiver. The second back-channel data includes the histogram contourparameter. The method additionally includes inserting the secondback-channel data to a second data stream by the second opticaltransceiver. The method also includes transmitting the second datastream from the second optical transceiver to the first opticaltransceiver. The method further includes detecting the secondback-channel data by the first optical transceiver. The method alsoincludes determining a first set of adjustments by the first opticaltransceiver based on the first set of measurements. The method includesapplying the first set of adjustments to an optical transmitter by thefirst optical transceiver.

It is to be appreciated that embodiments of the present inventionprovide many advantages over conventional techniques. Among otherthings, by measuring actual signal characteristics (e.g., histogram) ata receiving optical transceiver of the data communication path,adjustments made by a transmitting optical transceiver improve datatransmission quality better than existing techniques, where typicallyone-time factory settings are applied to optical transceivers.Histograms and histogram contour parameters are efficient and effectivewhen used in measuring signal quality. For example, adjustments such aswavelength control may be specific to the optical link and actualoperating conditions (e.g., temperature, interference, etc.), which areinformation unavailable when optical transceivers were manufactured. Itis therefore advantageous for the closed loop techniques provided by thepresent invention to use these information and hence improvedperformance.

Embodiments of the present invention can be implemented in conjunctionwith existing systems and processes. For example, the back-channel datacan be implemented to be compatible with existing communicationprotocols. Back-channel data are used by optical transceivers that arepreconfigured to use them, and optical transceivers that are notconfigured to use the back-channel data may simply ignore them. Inaddition, optical transceivers according to embodiments of the presentinvention can be manufactured using existing manufacturing equipment andtechniques. In certain implementations, existing optical transceiverscan be upgraded (e.g., through firmware update) to take advantage of thepresent invention. Histogram generation and calculations can beperformed by configuring existing DSP devices. There are other benefitsas well.

The present invention achieves these benefits and others in the contextof known technology. However, a further understanding of the nature andadvantages of the present invention may be realized by reference to thelatter portions of the specification and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1 is a simplified diagram illustrating optical transceiveraccording to an embodiment of the present invention.

FIG. 2 is a simplified diagram illustrating an encoded data frameaccording to embodiments of the present invention.

FIG. 3 is a simplified diagram illustrating an optical transmitter withback-channel data control according to embodiments of the presentinvention.

FIG. 4 is a simplified diagram illustrating an optical receiver 400according to an embodiment of the present invention.

FIG. 5 is a simplified diagram illustrating relationship between BER andMZ bias.

FIGS. 6A-6C provide histogram plots associated with received opticaldata signals. Each of the plots shows four peaks that correspond to fourPAM levels.

FIG. 7 is a plot illustrating average peak value associated withhistograms illustrated in FIGS. 6A-6C.

FIG. 8 is plot illustrating a quadratic fit of peak value vs. peaknumber, which is associated with the histograms illustrated in FIGS.6A-6C.

FIG. 9 is a plot illustrating linear fit of peak value offset againstpeak number squared.

FIG. 10 is a simplified diagram illustrating signal strength relative tosignal noise.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to communication systems and methods.In a specific embodiment, the present invention provides an opticalreceiver that receives a data stream from an optical transmitter. Theoptical receiver determines a histogram contour parameter using the datastream and inserts the histogram contour parameter into a back-channeldata segment, which is then transmitted to the optical transmitter. Theoptical transmitter changes its data transmission setting based on thehistogram contour parameter. There are other embodiments as well.

Most optical communication modules have some form of internal controlsystems to maintain the optical performance. For example, typicalcontrol parameters include optical power, wavelength, extinction ratio,and/or others. However, in most cases, conventional techniques for thetransmitting optical module to maintain these parameters rely on proxymeasurements. For example, transmitted optical power may be measured bya tap and photodiode, or extinction ratio may be inferred from amodulator bias. Unfortunately, these conventional techniques areinadequate. A difficulty is that these proxy measurements may notrepresent the actual transmission characteristics, and as a result thetransmitting optical path is not optimized.

In optical communication, another difficulty is that in an optical linesystem (including fiber optics, amplifiers,multiplexers/de-multiplexers, dispersion compensation, etc.), optimaltransmission parameters may not be constant and may in fact change dueto the line equipment or conditions. This may render the transmissionparameters even farther from optimal.

It is to be appreciated that embodiments of the present inventionprovide advantages over existing techniques. More specifically,embodiments of the present invention make use of digital signalprocessors (DSP) and forward error correction (FEC) modules on theoptical receive path. The inclusion of a DSP and FEC on the opticalreceive path within the module itself allows the receiving side todetermine the quality of the incoming optical signal. Additionally,embodiments of the present invention provide an advanced FEC encodingthat includes the ability to place additional digital informationalongside the transmitted data (“back-channel”), thereby allowing thereceive-side module to inform the transmitting-side module of thecurrent signal integrity. For example, the optical receiver generateshistograms based on the received optical signals. Using the histograms,optical receiver generates control data (e.g., histogram contourparameter and/or quadratic fit coefficient) that is transmitted back tothe optical transmitter. The optimal transmitter uses the control datato adjust and optimize data transmission parameters accordingly. Morespecifically, histogram information is used to determine to equalize andoptimize the signal-to-noise ratio (SNR) of different PAM levels for thepurpose of data transmission.

With DSP/FEC and advanced FEC encoding working together, a closed-loopsystem can be implemented, where the optical parameters of the transmitside can be tuned to optimally to reflect the current opticalconditions. The tuning parameters include, but not limited to,compensating for aging or environmental effects of optical equipmentfrom the transmitting optical module through to the receiving opticalmodule.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

FIG. 1 is a simplified diagram illustrating optical transceiveraccording to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown in FIG. 1,transceiver 100 includes an optical receiver 101 that interfaces with anoptical communication link and is configured to receive and processoptical communication signals. In various embodiments, optical receiverincludes various components, such as filter, transimpedance amplifier(TIA), fiber optic connectors, and others. Optical receiver 101 mayadditionally include optical transmission devices such as opticalamplifiers, optical attenuators, chromatic dispersion compensation(static or tunable), lengths of fiber, patch panels and patch cables,optical multiplexers, optical de-multiplexers, etc. Among otherfeatures, optical receiver 101 converts received optical signals toelectrical signals that can later be processed. The electrical signalsare then processed by various digital signal processors (DSP). Forexample, application specific integrated circuit (ASIC) 110 includes FECdecoder 111 and a back-channel detector 112.

It is to be appreciated that once back-channel data are detected fromthe incoming data stream, it is determined that the source of thereceived optical signals is compatible with the use of back-channel datafor adjusting its operating parameters. In various embodiments, the useof back-channel data is a part of a predetermined communication protocolthat two or more transceivers use. If back-channel data is not detectedfrom the received optical signals, the source of the received opticalsignals is not equipped to utilize back-channel data, and it would beunnecessary and even wasteful to perform signal measurements that are tobe embedded into back channel data. According to various embodiments,the receiver section of transceiver 100 is capable of measuring a levelhistogram based on the received signal.

ASIC 110 may also include a module for measuring and analyzing signalintegrity of the received signal (i.e., electrical signals convertedfrom the received optical signals). Signal integrity may be evaluated invarious signal measurements that include, but are not limited to overallsignal-to-noise ratio (SNR), individual PAM-4 level SNR, overall PAM-4histogram, optical eye diagram, and/or others. In additional to signalintegrity, data error rate associated with the incoming signal may beevaluated as well. For example, FEC decoder 111 determines error ratebefore performing error recovery. More specifically, FEC decoder 111 hasthe ability to calculate a bit error ratio (BER) prior to FEC errorrecovery. Depending on the implementation, BER can be calculated inseveral different ways, such as overall BER, individual lane BER,individual PAM-4 level BER (i.e., MSB BER, LSB BER), bit-transitionerror matrix (e.g., in PAM-4, BER for 0->1, 0->2, 0->3 and all otherlevel transitions), and/or other ways. In certain applications, BER isuseful for optimization, but histograms can also be used. For example,when BER is relatively flat (e.g., see FIG. 5 and explanation below)over a large range of settings, it is difficult to rely on BER forsetting adjustments.

The back-channel detection module 112 is configured to detect whetherthe received signals include back-channel data that can be used tooptimize data transmission performance. For example, the back-channeldata are embedded by the source of the received signals (e.g., anotheroptical transceiver or communication apparatus). In various embodiments,the back-channel detection module 112 is coupled to a control module115. The control module 115 is configured to adjust various operatingand transmission parameters of transceiver 100 based on the back-channeldata. For example, operating parameters include temperature, biassettings, multiplexer settings, wavelength, and others, which aredescribed below. It is to be appreciated that the back-channel detectionmodule 112 may be implemented as a part of the closed feedback loop(e.g., between two optical transceivers). That is, data are transmittedto a second transceiver over an optical communication link. The secondtransceiver includes DSP and FEC module that measure the signal quality(e.g., SNR) and data quality (e.g., BER), and the measurement resultsare embedded in the back-channel data that are transmitted back totransceiver 100. In various embodiments, signal quality and data qualityare determined by using histograms. The back-channel detection module112 detects the existence of the back-channel data, which are used bythe control module 115 to adjust operating parameters of transceiver100. Depending on the operating condition and specific implementation,there could be iterations of processes for changing parameters,receiving back-channel data reflecting the signal quality associatedwith the changed parameters, and changing parameters again. According toembodiments of the present invention, back-channel data includehistogram data associated with transmitted signal, and the histogramdata are used to determine optimal parameters for data transmission.

As shown in FIG. 1, ASIC 110 includes a histogram generation module 116.In various embodiments, histogram generation module 116 is implementedas a part of the ASIC 110 DSP. Histogram generation module 116 isconfigured to process the received signal and generate level histogramsbased on the received signals. In various embodiments, histogramgeneration module 116 executes mathematics on the histograms to generatehistogram contour parameters (e.g., quadratic fit coefficients forcertain applications) that can be used by the transmitter to optimizetransmission parameters.

It is to be appreciated that, as explained below, back-channel data canbe used to adjust not only transmitter parameters for outgoing data, butalso receiver parameters for processing incoming data. For example, backchannel data can be used to adjust how incoming optical signals areprocessed.

In various embodiments, control module 115 stores near-end parameters,which may be determined at the time when the transceiver 100 ismanufactured. Control module 115 analyzes the received back-channeldata, which reflects the actual conditions of data transmission, and theadjustment of operating parameters can be modifying the existingparameter based on the existing near-end parameters. In variousembodiments, adjustment of operating parameters involves synchronizingand using both existing near-end data and the back-channel data thatreflects conditions for actual data communication.

Transceiver 100 includes an FEC encoder 114 and a back-channel insertionmodule 113 as shown. For example, the FEC encoder 114 and theback-channel insertion module 113 are implemented as a part of the ASIC110. It is to be understood that while FEC decoder 111 and FEC encoder114 are shown as two functional blocks in FIG. 1, FEC decoder 111 andFEC encoder 114 may be implemented a single FEC module. Similarly,back-channel detection module 112 and the back-channel insertion module113 can be implemented as single back-channel module.

FEC encoder 114 is configured to perform FEC encoding for electricalsignals that are to be transmitted through the optical transmitter 102.For example, FEC encoder 114 is configured to perform different types oferror correction. Back-channel insertion module 113 is configured toinsert back-channel data into the outgoing data stream that is to betransmitted. As explained above, back-channel data include informationregarding the quality of received data, which pertains to transmissionparameters and settings of the transmitting transceiver that sends datato transceiver 100. It is to be appreciated that the back-channelinsertion module is capable of inserting and/or detecting, with highfidelity, additional digital information alongside and withoutinterfering with the transmitted data. For example, a predefined segmentof outgoing data stream is used to embed the back-channel data. Invarious embodiments, back-channel insertion module 113 inserts histogramlevels and/or histogram contours into back-channel data. As explainedbelow, histogram contours can be calculated as polynomial coefficientsof second order fit equations. At the far end, histogram contours areused in calibrating operating parameters (e.g., MZ bias point, heaterpower, etc.) that alters the modulation phase.

In FIG. 1, a close loop technique is used for optical communication,with an optical transmitter and an optical receiver. It is to beunderstood that close loop techniques that use back-channel foroptimizing data communication can be used in other types ofcommunication links as well, such as existing communication lines withcopper wires and/or other mediums.

FIG. 2 is a simplified diagram illustrating an encoded data frameaccording to embodiments of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown in FIG. 2, anexemplary FEC encoded frame can be divided into a most significant bits(MSB) section and a least significant bits (LSB) section. Both the MSBsection and the LSB section include their own respective headers. Forexample, the headers are 128 bits long. A header is then subdivided intoalignment marker region (0-63 bits), “mfas” region (64-71 bits),overhead region (72-120 bits), and “ecc” region (121-127 bits). It is tobe appreciated that overhead region stores back-channel data, whichincludes information related to quality (e.g., measured and/orcalculated) of received signals. For example, an optical transceiverthat is not equipped to take advantage of the back-channel data cansimply ignore and skip over the back-channel region.

Histogram information based on the received signal can be large. But itis to be appreciated that, as explained below, histogram contourparameters (or quadratic fit coefficents) can be relatively small. Byperforming calculations, the DSP function of the ASIC 110 caneffectively reduce histogram information to histogram contourparameters, which are small enough to be stored at the overhead region(e.g., 72-120 bits).

Now referring back to FIG. 1. Outgoing electrical signals are convertedto optical signals and transmitted by the optical transmitter 102. Forexample, optical transmitter 102 includes one or more lasers devices(e.g., laser diode with cooling), one or more modulators. Additionally,optical transmitter 102 may include multiplexing and optical controlblocks. Implementation and operating parameters of optical transmitter102 usually have significant impact on signal quality and datatransmission performance of the outgoing data stream. For example, oneof the operating parameters is modulator heater power, which affects themodulator bias. For example, the DAC controlled heater receives controlsignal from the control module, which uses histogram contour as a basisfor generating the control signal. It is to be noted that modulator biascan be changed in various ways, and an important aspect of usingoperating parameters is to change modulator bias and optimize datatransmission. In certain embodiment, heater power is configured in abranch of a modulator, which affects modulator operation. There areother operating parameters associated with modulator bias. By adjustingoperating parameters and settings of optical transmitter 102, signalquality and data transmission performance can be improved and optimized.While operating parameters and settings can be optimized initially atthe factory, being able to adjust these parameters and settings based onactual signal measurements is better, since actual signal measurementsreflect true operating conditions (e.g., fiber optic lines,interference, temperature, etc.).

According to various embodiments, the control module 115 of thetransceiver 100 processes the received back-channel data, which includeactual measurements of data quality as measured by a second transceiverthat receives data from transceiver 100. The control module 115 thendetermines the optical parameters and settings accordingly. For example,operating parameters and settings may include, but not limited to, thefollowing:

-   -   Modulator bias setting (e.g., heater power setting if a        thermo-optically controlled MZM); and    -   Setting for DAC controller heater;    -   Laser temperature setting (or TEC current if directly        controlled);    -   Laser bias current;    -   Multiplexer offset bias setting (e.g., heater power setting if a        thermo-optically controlled DLI).

As an example, back-channel data provides signal quality informationthat can be used to adjust parameters of laser devices. Morespecifically, laser devices used for optical data transmission may becontrolled using temperature and bias control parameters. FIG. 3 is asimplified diagram illustrating an optical transmitter with back-channeldata control according to embodiments of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown in FIG. 3, driver301 that generates driver signal based on outgoing data can be adjustedby a voltage swing parameter. Similarly, modulators 302 and 303 may beadjusted using settings such as RF amplitude, DC bias, and/or others.For example, modulators 302 and 303 may be implemented usingMach-Zehnder modulators (MZM). Light source for optical transmitter 300includes laser diodes 305 and 306. For example, laser diodes can beadjusted by changing laser bias and/or laser temperature. Similarly,delay line interferometer (DLI) 304, which functions as an opticalmultiplexer, can be adjusted with an offset bias and/or heatertemperature. It is to be appreciated that the control module 320 ofoptical transmitter 300 can use the histogram contour parameter todetermine which parameters (as listed above) are to be adjusted. Forexample, the control module 320 has a control interface that providescontrol signals for the abovementioned parameters such as bias control,temperature control, swing voltage, and others.

According to an embodiment, back-channel data are used as a part ofoptical receiver. As an example, optical receiver 101 is a part of thetransceiver 100 as shown in FIG. 1, and various operating parameters ofoptical receiver 101 may be adjusted based on back channel data.

FIG. 4 is a simplified diagram illustrating an optical receiver 400according to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. Optical receiver 400includes polarization beam splitter (PBS) 405 that splits the receivedoptical signal for processing. The received optical signal is thenprocessed by optical multiplexers 403 and 404. For example, multiplexers403 and 404 comprise DLI devices that can be adjusted using biassettings. For thermal-optically controlled DLIs, heater power settingsmay be used to adjust DLI operations. Multiplexers 403 and 404 arecoupled to photodetectors 401 and 402, whose current gain settings canbe adjusted. For example, photodetectors may be amplified, and thus gainsettings are needed. For example, avalanche photodiodes can be adjustedby changing its photocurrent gain. The outputs of photodetectors 401 and402 are coupled to TIA 406, which generates electrical signal based onthe received optical signals. Depending on the implementation, variousparameters such as amplitude, gain, and/or bandwidth, can be adjustedbased on back-channel data. As mentioned above, a control module 420 maybe used to process received back-channel data and generates controlsignals to adjust these parameters.

To make use of back-channel data, which include histogram relatedinformation, and to generate control signals for changing operatingparameters, a control module can be used. For example, abovementionedcontrol modules 420 may be implemented as a part of a computer engineblock, or a microcomputer that is a part of optical transceiver ASIC. Touse the transceiver 100 as an example, the control module is configuredwith the back-channel insertion module 113 to insert digital signalsalongside the transmitted optical data. More specifically, histogramcontour information is inserted into the back-channel data.Additionally, the control module is able to use the back-channeldetector 112 to detect back-channel data embedded in the receivedsignals. Once detected, the control module processes the back-channeldata and generates control signals accordingly. As explained above,back-channel data include histogram contour data, and the control moduleuses the histogram contour data as parameters for generating controlsignals. The control signals are used to adjust various operatingparameters of the transceiver (e.g., optical receiver, opticaltransmitter, etc.). For example, operating parameters include modulatorbias and temperature, as described above. Additionally, operatingconditions of the optical transceiver may change (e.g., interference,optical line quality, temperature change, etc.). By using back-channeldata, the control module adjusts and optimizes transceiver performanceaccordingly. Since the back-channel data are shared between two or moretransceivers, two transceivers form a feedback loop for optimizing datatransmission over a communication link.

The back-channel data used between two data transmission entitiesprovide a feedback mechanism for optimization data transmissionparameters. More specifically, receiving entity generates back-channeldata based on the quality of transmitted signal, and transmitting entityuses the back-channel data to adjust transmission parameters accordingly(multiple iterations may be needed). For the feedback mechanism to work,the back-channel data need to be available to the transmitting entity ina timely fashion, and the back-channel data must be meaningful indicatorof signal quality.

According embodiments of the present invention, operating parameters foroptical transmission are used for PAM transmission. In for example,off-quadrature biasing for MZ modulation can be used to maximize overallsignal-to-noise ratio (SNR) of four levels in PAM4 communication. Morespecifically, higher levels naturally have lower SNR, it is typicallydesirable compress lower levels and expand higher levels to obtainequalized levels with optimized SNRs. In the past, finding the optimalpoint can be done at BOL by scanning the phase angle and measuring forthe ideal bit-error ratio (BER), but over life the BER is a weak controlfor the optimal MZ bias point (heat power measured in mW), and is betterused to control other parameters (e.g., wavelength, DLI heater power).FIG. 5 is a simplified diagram illustrating relationship between BER andMZ bias. As can be seen in FIG. 5, for a wide range of MZ bias power,the BER curve is substantially flat and valued between 0.0002 and0.0003, and it is difficult to extrapolate the optimal MZ bias settingbased on the BER information. It is thus to be appreciated thataccording to various embodiments, the present invention takes advantagesof histogram to optimize data transmission. As an example, the optimalMZ bias setting (heater power measured in mW) is at about 35 mW, whichcorresponds to point 501 on the BER plot line.

As mentioned above, a receiving entity in an optical communicationnetwork is capable of generating and measuring a level histogram basedon the received data. Additionally, the receiving entity executesmathematics on the level histogram to generate “histogram contour”parameters. The histogram contour is then embedded into back-channeldata and transmitted back to the transmitting entity. The transmittingentity processes the histogram contour data from back channel and usingthe histogram contour data to adjust phase of MZ modulation (e.g., via aDAC controlled heater). The use of histogram is based on the observationthat the peak of histogram at each PAM level is inversely related tonoise of that level. For example, a higher PAM level can have largeramount of noise than a lower PAM level, and the SNRs of these two PAMlevels can still be the same.

The receiving entity (or near end entity) processes the received signalto generate the histograms. For example, histogram generation module 116in FIG. 1 calculates histogram by plotting bin number (e.g., ADC countproportional to signal level) versus hit count. FIGS. 6A-6C providehistogram plots associated with received optical data signals. Each ofthe plots shows four peaks that correspond to four PAM levels. TheX-axis is associated with modulation bias, and the Y-axis is BIN numberreadout (e.g., ADC readout count for the received optical signal). Plot6A shows histogram associated with 23 mW of power applied to themodulation heater; Plot 6B shows histogram associated with 33 mW ofpower applied to the modulation heater; Plot 6C shows histogramassociated with 40 mW of power applied to the modulation heater.

Histograms are useful, but they may be difficult to transmit. To makeuse of histograms, additional calculations are performed to obtain“histogram contour” data that can be inserted into the back-channel dataand used by the transmitter (e.g., far end optical transceiver). First,average values near each peak are calculated. FIG. 7 is a plotillustrating average peak value associated with histograms illustratedin FIGS. 6A-6C. Next, a second order fit is calculated with the Equation1 below:

y=c ₀ +c ₁ *x+c ₂ *x ²  Equation 1:

where: x=level number (0, 1, 2, 3, . . . ); and

-   -   y=average value near peak_x.

FIG. 8 is plot illustrating a quadratic fit of peak value versus peaknumber, which is associated with the histograms illustrated in FIGS.6A-6C. For example, the quadratic fit of peak values is based onEquation 1 above. The three lines in FIG. 8 are associated withcoefficients c₀, c₁, and c₁. In various embodiments, the first ordercoefficient c₁ of a quadratic fit over the level number is used. Incertain implementations, the zeroth order coefficient c0 of a linear fitover the square of the level number is used. It is to be understood thatthe coefficient that is to be transmitted may depend on the specificimplementation. It is to be noted that with histogram-based calibration,one or more parameters associated with the quadratic fit ofcoefficients, c₁ is most relevant in obtaining the optimal datatransmission parameters. In FIG. 8, there are three fit linesrespectively for coefficients c₀, c₁, and c₂, and the optimal settingfor data transmission is based on the point “c1₀”. As an example, theline “C1” is below 0 at 23 mW setting and above 0 at 40 mW setting. At33 mW setting (e.g., heating power that affects temperature and MZphase), the line “C1” is at or very close to 0 at point C1₀. As anexample, FIG. 8 shows that it is at the 33 mW MZ bias setting, SNRs forPAM4 levels are optimized and the data transmission BER is minimized. Itis to be appreciated that the 33 mW MZ bias setting, as determined usingc₁ coefficient of the quadratic equation, is very close to the 33.5 mWbias setting determined in FIG. 5. Therefore, for the purpose ofdetermining or interpolating the histogram to obtain histogram contourinformation, the first order coefficient c₁ of the second order fit canbe used. For example, c₁ value is used as the “histogram” contourparameter, where the quadratic equation is used. FIG. 9 is a plotillustrating linear fit of peak value offset against peak numbersquared. As can be seen in FIG. 9, c₁ value crosses zero at about 33 mWsetting.

The use of c₁ coefficient of quadratic fit is explained below. FIG. 10is a simplified diagram illustrating signal strength relative to signalnoise. In FIG. 10, P_(k) is the optical power level of the receivedsymbol “k”; N_(k) is the noise power associated with the received symbol“k”; T_(k) is the decision threshold between the two adjacent powerlevels of PAM power levels. For example, T_(k+1) is adjacent to T_(k),and T_(k) is adjacent to T_(k−1). For the purpose of illustration,assume decision thresholds T_(k) are positioned halfway between receivedoptical power levels. For example, T_(k−1) is positioned betweenP_(k+1)+N_(k+1) and P_(k)+N_(k). In an exemplary optical datacommunication system, the noise power is proportional to the receivedoptical power level, which can be expressed by Equation 2 below:

σ_(N) _(k) ² =c ₀ P _(k)  Equation 2:

The probably of error associated with symbol k (assuming Gaussian noise)can be expressed using Equation 3 below:

$\begin{matrix}{\mspace{79mu} {{{P\text{?}} = {{0.5\mspace{11mu} {{erfc}\left( \frac{P_{k} - P_{k - 1}}{\sigma_{k} + \sigma_{k - 1}} \right)}} + 1 - {0.5\text{erfc}\mspace{11mu} \left( \frac{P_{k} - P_{k + 1}}{\sigma_{k} + \sigma_{k + 1}} \right)}}}{\text{?}\text{indicates text missing or illegible when filed}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

where Erfc is the complementary error function (sometimes referred toGauss error function).

The total probability of error over M equiprobable symbols can then beexpressed using Equation 4 below:

$\begin{matrix}{P_{E_{total}} = {\frac{1}{M}{\sum_{k = 0}^{M - 1}P_{E_{sy{mbol}\mspace{11mu} k}}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

The total probability is minimized when the individual conditionalprobabilities PE for each symbol are equal. This implies that theconditional probabilities of error should not depend on the index k, or

$\frac{P_{k + 1} - P_{k}}{\sigma_{k} + \sigma_{k + 1}}$

should be independent of k. To equalize conditional probabilities,optical power levels can be quadratically spaced according to Equation 5below:

$\begin{matrix}{\mspace{79mu} {{P_{k} = {c\text{?}k^{2}}}\mspace{20mu} {\sigma_{Nk}^{2} = {c_{o}P_{k}}}\mspace{20mu} {\sigma_{k} = {\sqrt{c_{o}P_{k}} = {\sqrt{c_{o}c\text{?}}k}}}\mspace{20mu} {\sigma_{k + 1} = {\sqrt{c_{o}c\text{?}}\left( {k + 1} \right)}}\mspace{20mu} \begin{matrix}{\frac{P_{k + 1} - P_{k}}{\sigma_{k + 1} + \sigma_{k}} = \frac{c\text{?}\left( {\left( {k + 1} \right)^{2} - k^{2}} \right)}{\sqrt{c_{o}c\text{?}}\left( {\left( {k + 1} \right) + k} \right)}} \\{= \frac{c\text{?}\left( {{2\; k} + 1} \right)}{\sqrt{c_{o}c\text{?}}\left( {{2k} + 1} \right)}} \\{= \sqrt{\frac{c\text{?}}{c_{0}}}}\end{matrix}{\text{?}\text{indicates text missing or illegible when filed}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

As an example, for PAM4 communication with four levels, if levels arequadratically spaced, the level would be proportional [0, 1, 4, 9], andhence the spacing between the power levels is [1, 3, 5]. Normalized to“1”, the power level spacing would be [1/9, 3/9, 5/9], or about [0.11,0.33, 0.56]. It is to be appreciated that spacing arrangement among PAM4levels calculated here is consistent with the exemplary parameteroptimization described above.

It is to be appreciated that the use of first order coefficient foroptimizing data transmission, the actual optimization process areaccomplished by using histograms and histogram contour parameters (e.g.,quadratic fit) by the DSP function of transceivers. Depending on theactual data transmission system, other implementations are possible aswell.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. An optical receiver device comprising: an opticalinterface for receiving incoming optical signals from a transmitter; asplitter configured to split the incoming optical signals into a firstoptical signal and a second optical signal; a first optical multiplexerconfigured to multiplex the first optical signal to provide a firstmultiplexed optical signal, the first optical multiplex being configuredto operate using a first parameter; a second optical multiplexerconfigured to multiplex the second optical signal to provide a secondmultiplexed optical signal; a first photo detector configured to processthe first multiplex optical signal; an optical transimpedance amplifier(TIA) configured to convert an output optical signal from the firstphoto detector to a first electrical signal; and a control moduleconfigured to generating a plurality of parameters based using abackchannel data, the plurality of parameters including the firstparameter, the backchannel data being associated with a signal qualityvalue associated with a histogram characterizing performancecharacteristics of the transmitter, the signal quality value beingcalculated using a quadratic fit of a histogram and a first ordercoefficient of the quadratic fit.
 2. The optical receiver device ofclaim 1 wherein the performance characteristics include at least amodulator characteristic associated with an optical modulator of thetransmitter.
 3. The optical receiver device of claim 1 wherein: Thefirst optical multiplexer comprises a first delay line interferometer(DLI); the first parameter comprises a first bias setting.
 4. Theoptical receiver device of claim 3 further comprising a second photodetector configured to process the second multiplexed optical signal. 5.The optical receiver device of claim 4 wherein the first photo detectoris configured to operate using a second parameter, the second parametercomprising a current gain setting.
 6. The optical receiver device ofclaim 3 wherein the optical TIA is further configured to convert anoutput optical signal from the second photo detector to a secondelectrical signal.
 7. The optical receiver device of claim 1 wherein theoptical TIA is configured to operate using a second parameter, thesecond parameter comprising a gain setting.
 8. The optical receiverdevice of claim 6 wherein the control module is implemented using amicrocontroller.
 9. The optical receiver device of claim 1 wherein thecontrol module comprises an ASIC circuit.
 10. The optical receiverdevice of claim 1 wherein the control module is configured to detect thebackchannel data.
 11. The optical receiver device of claim 1 wherein thesignal quality value is associated a line quality of an optical channel.12. An optical receiver device comprising: optical interface forreceiving incoming optical signals from a transmitter; an opticalinterface configured to receive the incoming optical signals, theincoming optical signal comprising backchannel data; a control modulecomprising a detector for detecting the backchannel data and configuredto generating a plurality of parameters based using the backchanneldata, the plurality of parameters including a first parameter, thebackchannel data being associated with a signal quality value associatedwith a histogram characterizing performance characteristics of thetransmitter, the signal quality value being calculated using a quadraticfit of the histogram and a first order coefficient of the quadratic fit;a first delay line interferometer (DLI) configured to multiplex a firstoptical signal based on the incoming optical signals to provide a firstmultiplexed optical signal, the first optical multiplex being configuredto operate using the first parameter; a first photo detector configuredto process the first multiplex optical signal; and an opticaltransimpedance amplifier (TIA) configured to convert an output opticalsignal from the first photo detector to a first electrical signal. 13.The optical receiver device of claim 12 wherein: the plurality ofparameters further comprises a second parameter; the first photodetector is configured to operate using the second parameter.
 14. Theoptical receiver device of claim 12 further comprising a second DLIconfigured to operating using a second parameter.
 15. The opticalreceiver device of claim 13 further comprising a second photo detectorcoupled to the second DLI.
 16. A method for optimizing optical datatransmission, the method comprising: receiving an incoming opticalsignal from a transmitter; detecting backchannel data in the incomingoptical signal; obtaining a histogram contour parameter encapsulated inthe backchannel data, the histogram contour parameter being based on afirst order coefficient of a quadratic fit; generating a plurality ofparameters using at least the histogram counter parameter, the pluralityof parameters including a first parameter and a second parameter; andprocessing the incoming optical signal by a first multiplexer using atleast the first parameter.
 17. The method of claim 16 further comprisingprocessing the incoming optical signal by a second multiplexer using atleast the second parameter.
 18. The method of claim 16 processing theoutput of the first multiplexer by a photo detector using the secondparameter.
 19. The method of claim 16 wherein the histogram contourparameter is based on a PAM4 histogram.
 20. The method of claim 16further comprising converting the incoming optical signal to anelectrical signal using the second parameter.